You are right that it is rare, but it can be done. IMHO, my instructor is very good and has a 93% pass rate among her students. She is very thorough and teaches in a way that is helpful and easy to understand. She spends extra time with her students when they struggle and never signs you off for a checkride unless she is confident you can pass. This may contribute to the fact that she has four steady students. I was lucky to be assigned to her from the beginning.